Whenever that happen, first check if the obi rebooted. You can see this under ObiTalk. Go to the Obi Expert Configuration, then system status, then uptime. See how long it's been up. If it's been up for a while, yet you experienced the issue, then look at something other than the Obi for the cause. My wife has regularly made 30+minute plus calls with her family.
i.e. mine says today: 29 Days 9:48:08 (0)